So as an update, I went ahead and mixed Vallejo's US Forest Green with Intermediate Green to brighten it up. It worked a bit, at least enough to tell a difference between the two greens. I am happy with the results, just ahd to get out of my comfort zone and experiment a bit.
SAM_8853 by Darren Keller, on Flickr
SAM_8854 by Darren Keller, on Flickr
Also, don't mind the tan on the first picture, it got runny so I will need to touch it up again.